In the battle for the sky-highest fuel-efficiency figure, Volkswagen continues to hone its superweapon. The XL1 concept, unveiled today at the Qatar auto show—foolishly, we neglected to put a man on the scene—is a plug-in hybrid boasting triple-digit fuel economy. It builds on prior super-high-efficiency concepts from VW, namely the 1-Liter that debuted in 2002 and the L1 concept that appeared in 2009. The former used a tiny diesel engine and was designed to travel 100 kilometers on a single liter of fuel, hence the name; the L1 added a plug-in hybrid system in pursuit of a more marketable approach to the same goal.